Is (wild) grass good as a cover crop?

I understand all the benefits of covercrops, but is grass (just wild grass growing naturally) good as a cover crop?
If I read instructions for planting fruit trees, they always say to keep the grass away from the tree. Is there something special about grass or does this apply to all cover crops? Should I use mulch around young trees in stead of cover crops?

A lot of instructions say that you should keep the grass away, also around mature trees (e.g. with mulch), because grass competes for nutrients. But if I dont mow the grass, or mow and leave the grass on place to compost, the nutrients will be recycled in the soil, right? So no or maybe a little competition, but also a lot of benefits, right?

So what is so bad about grass and what is better about other cover crops?

My land is in Portugal and it is covered in wild grasses, with fruit trees, grape vines and olive trees in it. I like this natural cover crop, but should I plant some special cover crops around the trees? And what about young trees?

thanks,
Marie

I believe a lot of advice about grass around the base of trees is very "it depends".  Grass roots and tree roots can be feeding at different levels and so are complementary. You are not  apt in your environment have rodents that will use the cover of long grass and snow to eat the bark and kill the trees. That is often the reason for the advice in northern and high elevations sites with winter snow.
My preference is to mow around the tree with a scythe just before andy fruit will drop for the convenience of harvest and clean up. If you have trees and grass doing well together definitely don't disturb the symbiotic relationship because of advice given for a different habitat. It sound like you are in a savanna habitat not a forest habitat. So grass around the trees is apropriate.
If you are going to be mowing with a mulching mower then adding a legume might increase the nutrients that eventually reach the tree roots.

Tree roots (feeding roots) tend to remain in the top 24 inches (60 cm) of soil, these important roots also tend to be found from 1/2 way between the trunk and the drip line, out to around 60 cm beyond the drip line.

Younger trees benefit from little or no competition by other plants since their roots are closer to the trunk (see the above distance for feeder roots), that is why most people use a mulch layer in the early years of a tree's life.

Once a tree is well established, grasses pose no problem at all via competition for moisture or nutrients.

Rodents will tend to stay away from trees when there is nothing for them to hide in or under thus the removal of mulches once the tree is established.
You keep mulches at least 6" (10 cm) away from trunks partly for this reason, the other half of the reason is that moisture on a new trunk invites molds and fungi to attack young (thin) bark.

As a tree matures it normally pushes soil away from the trunk by the main support roots growing larger and displacing that soil along with rain water erosion at that point of juncture between tree and soil.


Redhawk